1. Field of the Invention
This invention involves a process for manufacturing a micropoint electron source with an auto-aligned focussing grid. Such a micropoint electron source can be used in particular in a device for visualisation by cathodoluminescence excited by field emission.
2. Discussion of the Background
Documents FR-A-2 593 953 and FR-A-2 623 013 disclose devices for visualisation by cathodoluminescence excited by field emission. These devices include a emitting cathode electron source with micropoint.
By way of illustration, FIG. 1 is a cross section view of such a micropoint viewing screen. In the interest of simplification, only a few aligned micropoints are shown. The screen is composed of a cathode 1, which is a plane structure, oriented with respect to another plane structure which forms the anode 2. The cathode 1 and the anode 2 are separated by a space in which a vacuum has been created. The cathode 1 includes a glass substrate 11 on which the conducting level 12 has been applied in contact with the electron emitting points 13. The conducting level 12 is covered with a layer of insulation 14, made of silica for example, which is itself covered by a conducting layer 15. Holes 18 of about 1.3 .cndot.m in diameter were made through the layers 14 and 15 up to the conducting level 12 to apply the points 13 on this conducting level. The conducting layer 15 acts as an extraction grid for the electrons which will be emitted by the points 13. The anode 2 includes a transparent substrate 21 covered by a transparent electrode 22 on which luminescent phosphors or luminophores 23 have been deposited.
The operation of this screen will now be described. The anode 2 is brought to a positive voltage of several hundred volts with respect to the points 13 (typically 200 to 500 V). A positive voltage of several dozens of volts (typically 60 to 100 V) with respect to the points 13 is applied to the extraction grid 15. Electrons are then drawn from the points 13 and are attracted by the anode 2. The trajectories of the electrons are within a half-angle cone at the peak .cndot., depending on various factors such as the shape of the points 13. This angle causes a defocusing of the electron beam 31 which increases as the distance between the anode and the cathode is increased. One way to increase the yield of the phosphors, and thus the luminosity of the screens, is to work with higher anode-cathode voltages (between 1,000 and 10,000 V), which implies separating the anode and the cathode further in order to avoid the formation of an electric arc between these two electrodes.
If good resolution on the anode is desired, the electron beam must be refocused. This refocusing is classically obtained with a grid which can either be placed between the anode and the cathode or placed on the cathode.
FIG. 2 illustrates the case where the focussing grid is placed on the cathode. FIG. 2 repeats the example of FIG. 1, but limited to a single micropoint for greater clarity in the drawing. An insulating layer 16 was applied to the extraction grid 15 and bears a metallic layer 17 which acts as a focussing grid. Holes 19 of an appropriate diameter (typically between 8 and 10 .cndot.m) and concentric to holes 18, were etched in layers 16 and 17. The insulating layer 16 electrically insulates the extraction grid 15 and the focussing grid 17. The focussing grid is polarised with respect to the cathode in order to give the electron beam the shape shown in FIG. 2.
Simulation calculations show that centering of the holes 19 of the focussing grid with respect to the holes 18 of the extraction grid is extremely important. This structure is generally made using the classic photoetching techniques used in microelectronics. For example, with a first level of photoetching, the holes 19 of the focussing grid are defined, then a second level of photoetching is used to make holes 18 in which the points will be placed. To ensure proper functioning, the second level must be positioned in an extremely precise manner with respect to the first level. This can only be done with very high-quality, expensive equipment, a serious drawback if large areas are treated. In addition, if the holes of the extraction grid are made by photolithography from a microsphere network, their arrangement is random, which rules out the use of a phototemplate for making the apertures of the focussing grid.